Variable radio frequency micro-electromechanical switch

ABSTRACT

A radio frequency micro-electromechanical switch (generally referred to using the acronyms RF MEMS) is described. Also described is a method of producing such an RF MEMS switch.

The present invention generally relates to a radiofrequencymicro-electromechanical switch (generally referred to using the acronymsRF MEMS) as well as to a method allowing such a switch to be produced.

A switch refers, within the meaning of the present invention, to anelectrical or electronic component that, under the effect of an outsidecommand, is capable of changing the electrical power level that itconveys over at least 2 distinct states.

The demand for reconfigurable radiofrequency (RF) components is growingendlessly following the increase in wireless data transfer needs.Indeed, the multiplication of telecommunications standards complexifiesthe architectures of devices and requires the integration ofreconfigurable components. RF MEMS switches are among serious candidatesmaking it possible to meet this need, in particular owing to their lowelectrical losses, highly linear behavior and low consumption relativeto traditional semiconductors.

These RF MEMS switches can be combined in the form of digital matrices,this combination making it possible to obtain a device having awell-defined and precise unitary variation, a high linearity of theelectrical response, and low electrical losses. Other technologies, suchas the stack of fixed-capacity MOS transistors, can be used for the samepurpose. These MOS transistors can be made at low costs and are easy tointegrate, but have a moderate quality factor Q, i.e., the inverse ofthe product of the serial resistance by the minimum capacity value thatit can reach is moderate. The higher this factor Q is, the betterperforming the switch is considered to be.

Thus, if one wishes to take advantage of higher quality factors of MEMSswitches while reducing manufacturing costs, the size of the switch andthe simplicity of production are critical aspects that must be improvedto supplant semiconductor technologies.

To date, many efforts to simplify the manufacturing method for RF MEMSswitches have been undertaken. Their integration into a standardthin-layer production chain (CMOS, for example) would allow a drasticreduction in production costs. Thus, patent application US20150235771describes a MEMS capacitance made by thin layers having an RF line, aswell as control electrodes inserted into the substrate, a MEMS membraneable to move when a voltage is applied on the control electrodes or theRF line, the MEMS membrane being inserted into a hermetic cavity, with acontrol electrode placed above the cavity and a dielectric layergripping the assembly. However, this configuration has the drawback ofinducing a high resistivity of the RF line and losses due to straycapacity. Furthermore, the production of this MEMS uses many processsteps, which makes it complex and expensive to manufacture.

In order to respond to the raised problem while avoiding theaforementioned drawbacks, the applicant has developed a radiofrequencymicro-electromechanical switch, comprising:

-   -   a semiconductor and/or insulating substrate having an        essentially planar face;    -   a first RF line capable of conveying an RF signal, said first RF        line comprising at least one metal layer, said first RF line        being arranged on said face of the substrate;    -   a second RF line capable of conveying an RF signal, said second        RF line comprising at least one metal layer;    -   a MEMS membrane able to be deflected by one or several        activations of the electrostatic type toward the substrate or in        the opposite direction, said MEMS membrane comprising at least        one layer of metal and being substantially parallel to the        substrate and being connected to the first RF line via one or        several anchors;    -   a dome, having an inner face across from said face of the        substrate and an outer face opposite said inner face and        comprising at least one dielectric layer, said dome being        arranged between the second RF line and the MEMS membrane,        hermetically encapsulating said MEMS membrane in a cavity (C),        and being anchored on said face of the substrate,        said switch being characterized in that said second RF line        comprises at least a first section in contact with said face of        the substrate and a second section adjacent and electrically        connected to said first section, said second section at least        partially covering the upper part of said dome.

In the switch according to the invention, the positioning of the secondRF line allows the use of RF lines having a greater thickness (forexample around 5 microns) relative to the RF lines of the state of theart. This greater thickness makes it possible to obtain very smallserial resistances, which increases the RF performances of thecomponent. Furthermore, this configuration makes it possible to create aswitch having low stray capacitances owing to the presence of an air gapbelow the RF lines.

The configuration of the switch according to the invention isessentially compact, and this compactness makes it possible to reducethe temperature sensitivity of the switch, limit the manufacturing costsand facilitate the integration of switch matrices into RF circuits, forexample.

The dome of the switch according to the invention may further be coveredby a discontinuous metal layer, covering its outer face.

Discontinuous, within the meaning of the present invention, refers to alayer comprising disjointed patterns (dots, lines, geometric shapes,etc.), which may or may not be connected to one another. Furthermore,some patterns may be connected to the first RF line or the second RFline. Advantageously, the MEMS membrane, which may have any shape, mayfurther comprise a dielectric layer and/or several additional metallayers. This dielectric may for example be chosen from the list made upof alumina, silicon oxide and silicon nitride.

Advantageously, the second section of the second RF line may be at leastpartially inserted into said dielectric layer forming the dome. Saidconfiguration may in particular make it possible to obtain a highercapacitance value when the membrane is deflected upward such that itcomes into contact with the lower surface of the dome.

Advantageously, the switch according to the invention further comprises:

-   -   one or several upper activation electrodes electrically        connected to one another and able to deflect said MEMS membrane        through an electrostatic activation, said upper activation        electrode being arranged on the outer face of the dome and/or        one or several central activation electrodes connected to one        another electrically and able to deflect said MEMS membrane        through an electrostatic activation, said central activation        electrode being arranged on the inner face of the dome and/or        one or several lower activation electrodes connected to one        another electrically and able to deflect said MEMS membrane        through an electrostatic activation, said lower activation        electrode being arranged on said face of the substrate in the        hermetic cavity.

The electrostatic-type activation of the MEMS membrane can therefore bedone by two different means:

-   -   the activation is done by an RF line:

-   in this case, a direct voltage is applied between the second RF    signal line and the membrane. This voltage creates an electrostatic    force that will deflect the MEMS membrane toward the second section    of the second RF line. When the voltage is lowered and it is no    longer sufficient to offset the return force, the membrane returns    to its initial position substantially parallel to the face of the    substrate, or    -   the activation is done by the activation electrodes:

-   when a direct electric voltage is applied between the electrodes and    the MEMS membrane, an electrostatic force is created and will    deflect the MEMS membrane toward the electrode. When the voltage is    lowered and it is no longer sufficient to offset the return force,    the membrane returns to its initial position substantially parallel    to the face of the substrate. This activation makes it possible to    control the mobile membrane independently of the RF signals.

Advantageously, the switch according to the invention may comprise oneor several upper activation electrodes, each of them being electricallyconnected to a central electrode by means of a metal via.

Advantageously, the switch according to the invention may comprise oneor several stop pins arranged in the cavity so as to prevent any contactbetween the central or lower activation electrodes and the MEMS membranewhen it is deflected. In this advantageous embodiment, this pin may belocated:

-   -   below the lower face of the dome: the pin may then limit the        deflection of the membrane and thus prevent any contact between        the MEMS membrane and the central activation electrodes,    -   on the face of the substrate: the pin may then limit the        deflection of the MEMS membrane and thus prevent any contact        between the MEMS membrane and the lower activation electrodes,    -   on the MEMS membrane: the pin may then limit the deflection of        the MEMS membrane and thus prevent any contact between the MEMS        membrane and the central activation electrodes,    -   below the MEMS membrane: the pin may then limit the deflection        of the MEMS membrane and thus prevent any contact between the        MEMS membrane and the lower activation electrodes.

The switch according to the invention may be used either as a switchedcapacitance, or as an ohmic switch.

In the case where the switch according to the invention is used as anohmic switch, the dome includes at least one opening in which a metalpin is housed that is formed in the extension of said second section ofthe second RF line, such that said MEMS membrane and said second sectionof the second RF line are able to come into contact when said MEMSmembrane is activated by an upper or central activation electrode so asto thus form an ohmic contact.

In the case where the switch according to the invention is used as acapacitance, the dome comprises at least one dielectric layer separatingthe MEMS membrane and the second section of the second RF line, so as toform a Metal-Dielectric-Metal capacitance. In this embodiment, a layerof metal can advantageously be arranged below said dielectric layer andcomes into contact with the MEMS membrane when said membrane isdeflected toward the dome.

The switch according to the invention can therefore be used either ascapacitance, or as ohmic contact, these embodiments each benefiting fromthe increase in the RF properties contributed by the positioning of saidsecond RF line on the upper part of the dome. In the case of thecapacitance, the variable distance between said MEMS membrane and thesecond section of the second RF line makes it possible to vary the valueof the electric capacitance and modifies the power insulation of thedevice. In the same way, when the switch is of the ohmic type, itinsulates RF current when the membrane is not activated and allows thecurrent to pass when it is activated, like a switch.

Furthermore, the dome can be hermetically sealed by the metal making upone of the RF lines or both RF lines and the cavity can contain a gas(for example air, N₂, Ar or O₂) or vacuum (primary or secondary vacuum).

The present invention also relates to a radiofrequencymicro-electromechanical microsystem (RF MEMS) comprising a switchaccording to the invention.

Lastly, the present invention also relates to a method for manufacturinga switch according to the invention, comprising the following steps:

-   -   a) depositing, on an essentially planar face of a semiconductor        or insulating substrate, a first sacrificial layer and producing        a pattern by removal (usually referred to as “lift-off”) and/or        etching a portion of said layer;    -   b) depositing, on said first sacrificial layer and on said        essentially planar face of the substrate, at least a first layer        of metal; and producing a pattern by lift-off and/or etching a        portion of said layer of metal, to form the first RF line and        the first MEMS membrane;    -   c) depositing, on said first RF line, a second sacrificial        layer; then producing a pattern by lift-off and/or etching a        part of said layer;    -   d) depositing, on said second sacrificial layer, a dielectric        layer, then producing a pattern by lift-off and/or etching a        portion of the dielectric layer, to form the dome having an        inner face across from said face of the substrate, an outer face        opposite said inner face, as well as one or several openings in        said dome;    -   e) eliminating the sacrificial layers through said openings; f)        depositing, on said outer face of said dome and on said        essentially planar face of the substrate, at least one second        metal layer; then producing a pattern making it possible to plug        said openings and forming the second RF line by lift-off and/or        etching of a portion of said second metal layer.

The second RF line thus formed comprises a first section in contact withthe essentially planar face of the substrate and a second sectionadjacent to said first section.

The openings formed during step d) are lateral openings, i.e., openingsthat are not across from the upper face of the MEMS membrane.

The elimination of the sacrificial layers can be done by dry etching orwet etching. In the case of wet etching, the MEMS membrane is containedin a liquid, which must go from the liquid state to the gaseous state:this transformation can be done by a critical point dryer (usuallyreferred to using the acronym CPD).

Other advantages and specificities of the present invention will emergefrom the following description, provided as a non-limiting example anddone in reference to the appended figures:

FIG. 1 shows a diagram of a switch according to the invention in topview (FIG. 1a ), in sectional view along line AA′ (FIG. 1b ) and insectional view along line BB′ (FIG. 1c );

FIG. 2 shows a schematic sectional view along line AA′ of a switchaccording to the invention in the case where it is used as capacitanceand where the second RF line is inserted partially into the dielectriclayer of the dome;

FIG. 3 shows a schematic sectional view along line AA′ of a switchaccording to the invention in the case where it is used as capacitanceand having two activation electrodes arranged on the dome and a metallayer arranged in the dielectric layer of the dome;

FIG. 4 shows a diagram of a switch according to the invention in thecase where it is used as ohmic contactor and has upper and centralelectrodes connected to one another, with a sectional view along lineAA′ (FIG. 4a ) and a sectional view along line BB′ (FIG. 4b );

FIG. 5 shows a schematic sectional view along line AA′ of a switchaccording to the invention in the case where it is used as capacitivecontact and has upper electrodes and a stop pin;

FIG. 6 shows a schematic sectional view along line AA′ of a switchaccording to the invention in the case where it is used as ohmic switchand has central and lower electrodes and a stop pin;

FIG. 7 shows schematic views of different successive steps a) to g) ofan embodiment of a switch according to the invention, with a sectionalview along line AA′ (FIG. 7a ) and a sectional view along line BB′ (FIG.7b ).

FIG. 1 shows a diagram of a switch according to the invention in topview. The first RF line 3 is electrically connected to the MEMS membrane5 by anchors 51, thus allowing an RF signal passing through the MEMSmembrane 5 to propagate in the first RF line 3. The second RF line 4 hasa first section 41 in contact with the face 21 of the substrate 2 and asecond section 42 partially covering the dome 6. These two sections areelectrically connected to one another, thus allowing an RF signalpassing through the first section 41 to propagate in the second section42 (FIG. 1a ).

The stack comprising the MEMS membrane 5, the dielectric (comprising thedielectric layer of the dome as well as any layer of air between themembrane 5 and the dielectric layer of the dome if the membrane 5 is notcompletely deflected), and the second section 42 of the second RF line 4forms the capacitance. The signal propagates from one RF line to theother through this stack. When the membrane 5 is deflected toward the RFline 4 and comes into contact with the dielectric dome, the capacitanceis higher. The switch according to the invention can therefore be usedas switched capacitance. In this particular case, the activation of themembrane is done by the RF line.

The dome 6 of FIG. 1 comprises at least one dielectric layer and iscovered by a layer of metal that can be discontinuous, the componentpatterns of which are connected to the first RF line 3 (FIG. 1b ). Thecomponent metal of the RF lines 3 and 4 makes it possible to guaranteethe hermiticity of the cavity.

The dome 6 has several anchor points 63 on the planar face 21 of thesubstrate 2 and three openings 64, 65 able to allow the elimination ofsacrificial layers S1, S2 having been used to develop the MEMS membrane5 and the dome 6 (cf. description of FIGS. 7a and 7b below): twoopenings 64 closed by the first RF line 3 (visible in FIGS. 1a and 1b )and an opening 65 closed by the second RF line 4 (visible in FIGS. 1aand 1c ). As shown in FIG. 1b (for the opening 64) and FIG. 1c (for theopening 65), these openings are lateral openings, which are not acrossfrom the upper face 51 of the MEMS membrane 5.

FIG. 2 shows a schematic sectional view of a switch according to theinvention in the case where it is used as capacitance and where thesecond RF line 4 is inserted partially into the dielectric layer of thedome 6. In this particular case, the second section 42 of the second RFline 4 is still separated from the MEMS membrane by at least onedielectric layer 8. The more deeply the RF line is inserted into thedome 6, the higher the maximum capacitance is, obtained when the MEMSmembrane 5 comes into contact with the dome 6.

FIG. 3 shows a schematic sectional view of a switch according to theinvention in the case where it is used as capacitance and where a metallayer 8 is arranged below the dielectric layer. The advantage of thismethod is allowing nearly perfect reproducibility of the switchedcapacitance subject to a slight degradation of the quality factor.

FIG. 4 shows a schematic sectional view of a switch according to theinvention in the case where it is used as ohmic contact and has upper 71and central 72 electrodes. In this particular case, each of the upperactivation electrodes 71 is connected to a central electrode 72 by ametal via 75 passing through the dome 6 (FIG. 4a ). The activationelectrodes are essential in the case of the ohmic contact, theactivation of the membrane not being able to be done via the RF linesthat come into contact.

The ohmic contact of FIG. 4 is done via a metal contact pin 91 passingthrough the dome and being in contact with the second RF line 4. Whenthe membrane is deflected, it comes back into contact with said metalpin and allows the RF currents to pass between the two RF lines (RFlines 3 and 4).

FIG. 5 shows a schematic sectional view of a switch according to theinvention in the case where it is used as variable capacitance and whereit has lower electrodes 73 and a stop pin 9. This pin here may either beplaced below the MEMS membrane 5 and in contact with said membrane or onthe face 21 of the substrate 2 and in contact with said face. When themembrane is deflected toward the lower electrodes 73, the pin limits thedeflection of the MEMS membrane 5 toward the lower electrodes 73,leaving an air gap between the MEMS membrane 5 and the lower electrodes73. Without said pin, the lower electrodes 73 could come into contactwith the membrane, which would charge the membrane 5 and cause thedevice to fail.

FIG. 6 shows a schematic sectional view along line AA′ of a switchaccording to the invention in the case where it is used as ohmic contactand has central 72 and lower 73 electrodes. The activation electrodes71, 73 cannot deflect the membrane 5 toward them. Thus, adding lowerelectrodes 73 makes it possible to deflect the membrane 5 toward thesubstrate 2 and increase the amplitude of the variations in electricproperties of the device.

FIG. 7 shows schematic views of the different successive steps a) to g)to produce a switch according to the invention, with a sectional viewalong line AA′ (FIG. 7a ) and a sectional view along line BB′ (FIG. 7b).

In FIGS. 7a and 7b , the diagrams corresponding to step (a) show a firstsacrificial layer S1 deposited on the substrate 2 after shaping thereof.

In FIGS. 7a and 7b , the diagrams corresponding to step (b) show a firstmetal layer M1 deposited on the first sacrificial layer S1. This firstmetal layer M1 is shaped by etching (dry or wet) to create the first RFline 3 and the MEMS membrane 5, these two components being electricallyconnected to one another by the anchors 51 of the MEMS membrane.

In FIGS. 7a and 7b , the diagrams corresponding to step (c) show thesecond sacrificial layer S2 after shaping thereof.

In FIGS. 7a and 7b , the diagrams corresponding to step (d) show thedielectric layer after shaping thereof to create the dome 6. The dome 6is anchored in the substrate 2 and allows the first RF line 3 to pass inorder to allow the connection with the MEMS membrane 5.

The openings 64, 65 allow the dry etching or wet etching of thesacrificial layers, wet etching requiring an additional step forcritical point dryer.

In FIGS. 7a and 7b , the diagrams corresponding to step (e) show theresult of the step for eliminating the sacrificial layers.

In FIGS. 7a and 7b , the diagrams corresponding to step (f) show thatstep f) is a step for depositing a second metal layer M2, said layerbeing intended to serve as a base for forming the different patterns ofthe following step.

As illustrated in the diagrams corresponding to step (g) of FIGS. 7a and7b , said second metal layer M2 is shaped by lift-off and/or etching(dry or wet) in order to create the second RF line 4 and to close theopenings 64, 65 formed during the preceding step, in a manner. Thissecond RF line 4 is broken down into a first section 41 in contact withthe planar face 21 of the substrate 2, and a second section 42 adjacentto the first section 41 (i.e., that is electrically connected to it). Atleast one of said second RF line 4 and said first RF line 3 closes thelateral openings 64, 65, thus creating a hermetic cavity C thatencapsulates the MEMS membrane.

1. A radiofrequency micro-electromechanical switch, comprising: at leastone of a semiconductor or insulating substrate having an planar face; afirst RF line capable of conveying an RF signal, the first RF linecomprising at least one metal layer, the first RF line being arranged onthe planar face of the substrate; a second RF line capable of conveyingan RF signal, the second RF line comprising at least one metal layer; aMEMS membrane able to be deflected by at least one activation of theelectrostatic type in one direction among a direction toward thesubstrate and a direction opposite the substrate, the MEMS membranecomprising at least one layer of metal and being parallel to thesubstrate and being connected to the first RF line at least one anchors;a dome, having an inner face across from the planar face of thesubstrate and an outer face opposite the inner face, comprising at leastone dielectric layer, the dome being arranged between the second RF lineand the MEMS membrane, hermetically encapsulating the MEMS membrane in acavity, and being anchored on the planar face of the substrate, whereinthe second RF line comprises at least a first section in contact withthe face of the substrate, and a second section adjacent andelectrically connected to the first section, the second section at leastpartially covering the upper part of the dome.
 2. The switch accordingto claim 1, wherein the MEMS membrane further comprises at least onedielectric layer.
 3. The switch according to claim 1, wherein the secondsection of the second RF line is at least partially inserted into thedielectric layer of the dome.
 4. The switch according to claim 1,further comprising at least one of: at least one upper activationelectrodes electrically connected to one another and able to deflect theMEMS membrane through an electrostatic activation, the at least oneupper activation electrode being arranged on the outer face of the domeat least one central activation electrodes connected to one anotherelectrically and able to deflect the MEMS membrane through anelectrostatic activation, the at least one central activation electrodebeing arranged on the inner face of the dome at least one loweractivation electrodes connected to one another electrically and able todeflect the MEMS membrane through an electrostatic activation, the atleast one lower activation electrode being arranged on the face of thesubstrate in the hermetic cavity.
 5. The switch according to claim 4,comprising at least one upper activation electrodes, each upperactivation electrode being electrically connected to a central electrodeby means of a metal via.
 6. The switch according to claim 4, comprisingat least one stop pins arranged in the cavity so as to prevent anycontact between at least one of the central and lower activationelectrodes and the MEMS membrane when the MEMS membrane is deflected. 7.The switch according to claim 4, wherein the dome includes at least oneopening in which a metal pin is housed that is formed in the extensionof the second section of the second RF line, such that the MEMS membraneand the second section of the second RF line are able to come intocontact when the MEMS membrane is activated by an upper or centralactivation electrode so as to thus form an ohmic contact.
 8. The switchaccording to claim 1, wherein the dome comprises at least one dielectriclayer separating the MEMS membrane and the second section of the secondRF line, so as to form a Metal-Dielectric-Metal capacitance when themembrane is activated and in contact with the dome.
 9. The switchaccording to claim 8, wherein a layer of metal is arranged below thedielectric layer and comes into contact with the MEMS membrane when theMEMS membrane is deflected toward the dome.
 10. A radiofrequencymicro-electromechanical microsystem comprising a switch as definedaccording to claim
 1. 11. A method for manufacturing a switch,comprising the following steps: a) depositing, on a planar face of atleast one of a semiconductor and an insulating substrate, a firstsacrificial layer and producing a pattern by at least one of lift-offand etching of a portion of the first sacrificial layer; b) depositing,on the first sacrificial layer and on the planar face, at least a firstlayer of metal; then producing a pattern by at least one of lift-off andetching a portion of the layer of metal, to form the first RF line andthe first MEMS membrane; c) depositing, on the first RF line, a secondsacrificial layer; then producing a pattern by at least one of lift-offand etching a part of the second sacrificial layer; d) depositing, onthe second sacrificial layer, a dielectric layer; then producing apattern by at least one of lift-off and etching a portion of thedielectric layer, to form the dome having an inner face across from theplanar face of the substrate, an outer face opposite the inner face, aswell as at least one openings in the dome; e) eliminating the first andsecond sacrificial layers through the openings; then f) depositing, onthe outer face of the dome and on the planar face of the substrate, atleast one second metal layer; then producing a pattern making itpossible to plug the openings and forming the second RF line by at leastone of lift-off and etching of a portion of the second metal layer. 12.The switch according to claim 1, wherein the MEMS membrane furthercomprises at least one additional metal layer.